In recent years, on the front lines of health care, it has been put into practice that a video of an endoscope or the like is displayed as a three-dimensional image on a display device during surgery, so that a surgeon is provided with more easily-comprehensible video. The three-dimensional image can be shown, as described in Patent Literature 1, for example, by using a head mounted display, which is a display device worn by a surgeon, and causing a shift to occur between a left-eye image signal and a right-eye image signal. Further, by letting a surgeon to wear polarized glasses having a polarization property of enabling three-dimensional viewing, the surgeon is allowed to perform three-dimensional viewing of a video displayed on a monitor or the like which is installed nearby.